This invention relates to a valve train for an internal combustion engine.
In valve trains of this kind, which are known, for example, from German. Patent 3,800,347, the first rocker lever or levers is/are pressed against the first cam or cams by the valve spring of the associated valve, while a separate spring is provided to press the second rocker lever against its cam. This spring must be designed so that the second rocker lever rests against its cam continuously over the entire rotational speed range, i.e. both during the base circle phase and during the lift phase, for which purpose the spring force required corresponds approximately to that of the valve spring. Due to the relatively large, continuously acting contact force, increased wear occurs on the second cam and on the second rocker lever. In order to reduce this wear, a known proposal (EP-A 264 253) is to provide two springs of different strengths arranged in series and supported on one another for the purpose of pressing the second rocker lever against its cam, the stronger spring being relaxed and the rocker lever being pressed against its cam only by the weaker spring during the base circle phase. In the lift phase, the weaker spring is first of all compressed and then the stronger spring is compressed via the weaker spring, with the result that the second rocker lever is then acted upon by a large contact force, which is required in order to ensure contact of the second rocker lever with its cam during the lift phase. The disadvantage here is that, during the transition from the base circle phase to the lift phase, only a fraction of the spring force of the stronger spring is initially effective after the compression of the weaker spring, since it was previously in a relaxed state and, assuming the normal linear spring characteristic, only develops its full spring force with a certain compression, as a result of which it is necessary to have a very stiff spring with a high spring rate which can lead to an excessive contact force at the maximum lift.
It is the object of the invention, in a valve train of the generic type, to achieve a reduction in the friction and wear of the second rocker lever against its cam.
This object is achieved according to the invention by virtue of the fact that the spring rests directly or indirectly, under prestress, against a fixed stop, from which it is only raised after the cam base circle phase has been passed through by the second rocker lever.